Recently, in accordance with rapid development and popularization of liquid crystal display (LCD) or the like, the demand for a cellulose ester (cellulose acylate) film, in particular, a triacetyl cellulose (TAC) film used as a protective film for the LCD or the like, has been increasing. According to the increase in the demand for the TAC film, the improvement in productivity thereof has been desired. The TAC film is produced as follows. A dope containing the TAC and a solvent is cast through a casting die onto a support continuously moving to form a casting film thereon. The casting film is dried or cooled to be solidified. The solidified casting film is peeled from the support to form a wet film. The wet film is dried and wound as a film. According to a solution casting method described above, it is possible to form a film containing less foreign materials and having more excellent optical properties in comparison with a film forming method by melt-extrusion.
In the solution casting method, as the support onto which the dope is cast, a band and a drum are used. The drum can achieve improvement in a casting speed more readily in comparison with the band. In order to solidify the casting film on the support, the casting film is dried, or cooled so as to turn into gel. Additionally, in order to adjust the optical properties of the TAC film, in particular retardation thereof, the TAC film is stretched.
The optimum film producing speed and the optimum stretching speed are different from each other. The film producing speed requires more time than the stretching speed. Therefore, in the case that the stretching speed is adjusted in accordance with the film producing speed, it is impossible to stretch the film enough to improve the optical properties. Accordingly, it is proposed that the film is stretched in an off-line manner independently from a solution casting line (see Japanese Patent Application Laid-open No. 2002-311240, for example).
In a case where the film is stretched in an off-line manner independently from the solution casting line as disclosed in Japanese Patent Application Laid-open No. 2002-311240, for the purpose of efficiently performing the stretching process, the TAC film is preferably stretched continuously. In this case, in order to supply the film continuously, a portion in which a rear end of a preceding polymer film and a front end of a trailing polymer film are overlapped with each other is heated to be welded. Thereby, the preceding film and the trailing film are spliced to each other (see Japanese Utility Model Applications Laid-open No. 53-020268 and 53-051864, for example).
However, as disclosed in Japanese Utility Model Applications Laid-open No. 53-020268 and 53-051864, when a heater is disposed above or below the portion in which the preceding and trailing polymer films are overlapped with each other, in the case that the temperature of the heater is raised or in the case that the contact time between the heater and the film is increased for the purpose of enhancing the welding, the temperature of the heated film exceeds the degradation temperature of the polymer in the film in some cases. The polymer film, in particular, cellulose ester film has the melting point of 300° C. and the decomposition temperature of more than 300° C. Since the melting point and the decomposition temperature are close to each other, in the case that the welding temperature is set higher for the purpose of welding the film sufficiently, pitting occurs on the welded portion of the polymer films or the degradation of the polymer proceeds due to the overheating of the polymer film. Accordingly, there is a problem that it is impossible to achieve sufficient splicing strength.
In an off-line stretching device, the TAC film is subjected to a splicing process for splicing a preceding film and a trailing film, a stretching process for stretching the spliced TAC film by holding the side ends thereof with use of plural clips in a film width direction while heating the TAC film continuously transported, and a relaxation process for subjecting the TAC film thus stretched to heat treatment for stress relaxation (for example, see U.S. application Ser. No. 12/058,631 corresponding to Japanese Patent Application No. 2007-084424). In the splicing process, as an easy splicing device, a single-sided splicing tape is used. In general, the single-sided splicing tape has a sheet as a base and an adhesive layer formed on one surface of the base. The base material of the base is different from that of the TAC film.
Further, the TAC film subjected to the stretching process and heat treatment for stress relaxation in the off-line stretching device shrinks in the film width direction after the heat treatment for stress relaxation. The thermal shrinkage amount of the base of the single-sided splicing tape and the thermal shrinkage amount of the film are different from each other. Therefore, as shown in FIG. 11, in the case that a preceding film 3a and a trailing film 3b are overlapped with each other and a single-sided splicing tape 60 is adhered thereon so as to extend in a film width direction B in order to splice the preceding film 3a and the trailing film 3b, the shrinkage amount of a spliced area 61 is larger than that of an area around the spliced area 61 (peripheral area). As a result, wrinkles 62 occur in the spliced area 61.
When the wrinkles 62 occur in the spliced area 61, the wrinkles 62 also occur in the peripheral area. The area having the wrinkles 62 can not be used as a product and need to be discarded. Therefore, there is a problem that the area as a product is decreased. Further, in some cases, the TAC film breaks due to the occurrence of wrinkles 62, thus causing a problem. The above-described problems are not limited to the TAC film, and also applied to other polymer films.
Accordingly, there is a problem that it is not possible to obtain sufficient splicing strength in the case that the splicing is performed by welding, and further wrinkles occur on the film which is subjected to welding, splicing, stretching, and the heat treatment for stress relaxation.